In the past, a variety of methods have been used to measure the thermal energy content of a fluid flowing through a conduit. Methods used in the past have involved the separate sensing of flow rate and temperature (relative to some reference such as ambient temperature) and, from these two independently measured variables, computing the thermal energy flow. Generally, the thermal unit used in the past has been the British thermal unit (Btu) which in practical use is defined as the amount of heat required to raise one pound of water one degree Farenheit.